Communication scrambler system

ABSTRACT

A scrambler system in which amplitude zero-crossings in speech are encrypted at a transmitter by combination with a pseudorandom digital sequence, the reverse process occurring at the receiver. The amplitude envelope need not be encrypted, but if desired, can be encrypted by combination with a further pseudorandom digital sequence. A higher level of scrambling can be achieved in the transmitted signal by using the conjugate of the amplitude envelope. With a sequence bit rate of about two kilobits per second, there is no increase in bandwidth requirements compared to the transmission of unscrambled speech. Thus standard transceivers can be employed, and pre-existing ones readily modified for scrambling and unscrambling.

United States Patent [191 Reynolds COMMUNICATION SCRAMBLER SYSTEM [75]Inventor: .lohn Spackman Reynolds,

Portsmouth, England [73] Assignee: The General Electric Company Limited,London, England [22] Filed: Apr. 12, 1972 [21] Appl. No.: 243,281

[52] U.S. Cl 325/32, 178/22, 179/15 R, 179/ 1.5 S [51] Int. Cl. H04k1/02 [58] Field of Search 325/32, 42, 44; 179/1.5 R; 178/22 [56]References Cited UNITED STATES PATENTS 3,560,659 2/1971 Greefkes et al325/42 3,123,672 3/1964 Ross 179/15 R 3,696,207 10/1972 Lundin et a1.179/l.5 S 3,659,046 4/1972 Angeleri et al. 325/32 3,614,316 10/1971Andrews, Jr. et a1 178/22 [4 1 Apr. 30, 1974 Primary ExaminerMaynard R.Wilbur Assistant ExaminerI-l. A. Birmiel Attorney, Agent, orFirm-Kirschstein, Kirschstein Ottinger & Frank ABSTRACT A scramblersystem in which amplitude zero-crossings in speech are encrypted at atransmitter by combination with a pseudo-random digital sequence, thereverse process occurring at the receiver. The amplitude envelope neednot be encrypted, but if desired, can be encrypted by combination with afurther pseudorandom digital sequence. A higher level of scrambling canbe achieved in the transmitted signal by using the conjugate of theamplitude envelope.

With a sequence bit rate of about two kilobits per second, there is noincrease in bandwidth requirements compared to the transmission ofunscrambled speech. Thus standard transceivers can be employed, andpre-existing ones readily modified for scrambling and unscrambling.

20 Claims, 4 Drawing Figures 68 list Pseudo-Random Sequence, GeneratorCOMMUNICATION SCRAMBLER SYSTEM This invention relates to communicationsystems and to apparatus for use in such systems.

The invention relates more particularly but not exclusively tocommunication systems and apparatus for the transmission and receptionof speech.

Personal wireless transceivers are now widely used by policeman, forexample while on a routine patrol, to maintain contact with theirheadquarters. However, the value of such communication systems islessened by their vulnerability to eavesdropping by unauthorisedpersons.

Scramblers employing various forms of frequency shifting arrangementsare known and have been widely employed. However, the time required byan unauthorised person having suitable equipment to unscramble afrequency-shift scrambled message is not very great, so that only alimited security is available. Transmissions scrambled on a truly randombasis are undecodable, but the arrangements required are impracticablefor widespread, everday use, wherein in any event, only, say, one hourof security is required.

It is therefore an object of the invention to provide an improvedcommunication system and apparatus for use in such a system which allowsat least a limited period of privacy or security.

According to a first aspect of the invention, in a communication systemfor transmitting the intelligence of an input signal, which said inputsignal is either a speech signal or other signal having components ofvariable frequency and variable amplitude, a transmitting terminal ofthe system has means to effect scalar combination of the input signal orof a signal which is derived from the input signal and which containsinformation as to at least the frequencies of higher frequencycomponents of the input signal with a digital signal having a pluralityof possible amplitude levels which occur in a pseudo-random sequence,the signal resulting from the combination or a signal derived therefromconstituting the signal transmitted to a receiving terminal of thesystem, this transmitted signal carrying information in respect both ofthe frequencies of said higher frequency components of the input signal(albeit masked by the digital signal) and of the amplitude envelope ofthe input signal, and the receiving terminal of the system has means toeffect scalar combination of the received signal or of a signal derivedfrom the received signal and a digital signal that is similar to thatutilised at the transmitting terminal and has the same pseudo-randomsequence to recover a first signal carrying information of at least thehigher frequency components of the input signal without digital masking,means to recover a second signal carrying information as to theamplitude envelope of the input signal, and means to reconstitute fromsaid first and second signals an output signal which is an intelligibleapproximation to the input signal.

The signal which is derived from the input signal may be derived byhigh-pass filtering or differentiation of the input signal.

The signal which is combined with the digital signal may be converted toa signal having a similar or the same plurality of amplitude levels asthe digital signal.

The information contained in the transmitted signal in respect of theamplitude envelope may be or be derived from a signal representing theamplitude envelope or its conjugate, and the last-mentioned signal maycontain a further digital signal having a plurality of possibleamplitude levels which occur in a further pseudorandom sequence, themeans to recover the second signal being such as to enable the maskingeffect of the further digital signal to be counteracted.

According to a second aspect of the invention, in transmitter terminalapparatus for transmitting the intelligence of an input signal, whichsaid input signal is either a speech signal or other signal havingcomponents of variable frequency and variable amplitude, the apparatuscomprises means to form a digital signal having a plurality of possibleamplitude level which occur in pseudo-random sequence, means to effectscalar combination of the digital signal with the input signal or with asignal which is derived from the input signal and which containsinformation at to at least the frequencies of higher frequencycomponents of the input signal, and means for including in thetransmitted signal, which is formed by or derived from the output of thelast mentioned means, information in respect of the amplitude envelopeof the input signal.

The transmitter terminal apparatus preferably includes means forlimiting the bandwidth of the signals to be transmitted to a bandwidthnot substantially more than the bandwidth of the input signal.

According to a third aspect of the invention, in receiver terminalapparatus for receiving a transmission of the intelligence of an inputsignal which said input signal is either a speech signal or other signalhaving components of variable frequency and variable amplitude, thetransmission being formed by or derived from a scalar combination of adigital signal having a plurality of possible amplitude levels whichoccur in pseudorandom sequence, with the input signal or with a signalderived from the input signal and which contains information as to atleast the frequencies of higher frequency components of the inputsignal, the transmission including information in respect of theamplitude envelope of the input signal, the apparatus comprises means toform a digital signal similar to that utilised in the formation orderivation of the transmission and having the same pseudo-randomsequence, means to effect scalar combination of the digital signal withthe received signal or a signal derived therefrom to recover a firstsignal carrying information of at least the higher frequency componentsof the input signal without digital masking, means to recover a secondsignal carrying information as to the amplitude envelope of the inputsignal, and means to reconstitute from said first and second signals anoutput signal which is an intelligible approximation to the inputsignal.

The transmitter terminal apparatus may include means for generating afurther digital signal having a plurality of possible amplitude levelswhich occur in a further pseudo-random sequence, and means for combiningor superimposing the further digital signal with or on the signalrepresenting the amplitude envelope or its conjugate, the receiverterminal apparatus including means for generating a further digitalsignal which is similar to the further digital signal utilised in thetransmitter terminal apparatus and having the same pseudorandonnsequence, means for affecting scalar combination of the further digitalsignal with said second signal to reconstitute the amplitude envelope orits conjugate.

The digital signals and further digital signals may have two, three,four, or more amplitude levels.

By limiting the bandwidth of the signals to be transmitted to thebandwidth of the original input signals (e.g. speech) the originaltransmitter and receiver of a pre-existing communication system may beused, i.e., an existing radio transceiver may readily be converted tothe form of the present invention, without exceeding its bandwidthcapabilities. Thus, for example, the transmitter terminal apparatus canfunction between the microphone and the remainder of an existing radiotransmitter and likewise the receiver terminal apparatus can functionbetween a standard radio receiver and its audio output stage.

Pseudo-random sequence generators of the feedback shift register typeare readily adaptable to give different output sequences, by changingthe points from which the feedback is taken. Thus code changing is easy.The sequence generators at the transmitter terminal and receiverterminal can be synchronised by breaking the feedback path of thereceivers sequence generator, (or the appropriate one of the two, as thecase may be) feeding an intelligence-free (i.e., speech-free) receivedsignal in to the shift register in place of the feedback, comparing theinput and the feedback, and after an identity in the comparison for anadequate period of time (equal to the time required to prime theregister with the sequence portion being transmitted, plus a safetymargin), reconnecting the feedback and disconnecting the received signalwhich can then be decoded by the now synchronised pseudo-random sequencegenerator. (An unsynchronised receiver sequence generator or one notproducing a correct sequence will cause the receiver output to beunintelligible).

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings wherein:

FIG. 1 is a block schematic diagram of a first embodiment of transmitterterminal apparatus in accordance with the invention;

FIG. 2 is a block schematic daigram of receiver terminal apparatus foruse with the transmitter terminal apparatus of FIG. 1; I

FIG. 3 is a block schematic diagram of a second embodiment oftransmitter terminal apparatus in accordance with the invention; and

FIG. 4 is a block schematic diagram of receiver terminal apparatus foruse with the transmitter terminal apparatus of FIG. 3.

Referring first to FIg. 1, transmitter terminal apparatus in accordancewith the invention comprises a microphone for converting an acousticinput signal such as speech to an electric signal representative of theintelligence to be transmitted, a differentiator 12 which enhances thehigh frequency components of the signal, and thereby increases the rateof zero crossing in the signal, for a purpose to be explained hereafter,and a high gain saturating amplifier or Schmitt trigger 14 whose outputis a binary signal and is fed as one input of a module-two adder 16(which may conveniently be considered as an "exclusive OR" circuit). Theother input to the module two adder 16 is a digital signal having apseudo-random binary sequence from a sequence generator 18. The outputof the difierentiator 12 is passed through an envelope amplitudedetector comprising a rectifier 20 and a low pass filter 22, the outputof which is a unidirectional signal the level of which at any instant isa measure of the amplitude envelope of the data to be transmitted. Thelatter signal, and the output of the modulo two adder 16 are combined ina suitable signal combination circuit 24, and subsequently broadcast bya conventional wireless transmitter 26. The circuit 24 preferablyamplitude modulates the signal supplied by the adder 16 with the signalsupplied by the filter 22 through useful results can be obtained bylinear addition.

With speech having a band-width in the range 300 Hz to 3 kHz and theoutput of the sequence generator 18 being within this range, for exampleabout 2,000 bits per second, the band-width of the signals reaching thetransmitter 26 can be restricted to lie within the above band withoutany serious loss of information. For this purpose, a low-pass orband-pass filter (not shown) may be connected between the circuit 24 andthe transmitter 26. Alternatively, the transmitter 26 may be constructedor adapted to accomplish the necessary bandwidth limitation. Thus nospecial frequency capability is required of the transmitter 26, whichmay be any conventional form of transmitter. The transmitted signal,though possessing speech like qualities in terms of waveform, iscompletely unintelligible without knowledge of the pseudo-randomsequence. Although the en velope of the speech input is not scrambled,it contains substantially no meaningful intelligence by itself.

Referring now to FIG. 2, receiver terminal apparatus for receiving thetransmissions of the arrangement of FIG. 1 comprises a wireless receiver30 which is compatible with the transmissions of the transmitter 26. Theoutput of the receiver 30 is fed both to a high gain limiting amplifieror Schmitt trigger 32 and an envelope amplitude detector comprising arectifier 34 and a low pass filter 36.

The outpt of the amplifier 32 is a binary signal whose level transitionscorrespond to the level transitions in the combined signal which is theoutput of the modulo two adder 16 (FIG. 1). Thus the amplifier 32creates a homologue of the combined signal of the transmitter. Thishomologue is fed to a modulo two adder 38 as one input thereof, theother input to the adder 38 being a digial signal comprising apseudo-random binary sequence from a sequence generator 40, and which isinstantaneously identical to the sequence being produced by the sequencegenerator 18 (FIG. 1). (The achievement of this identity will beexplained hereafter).

The output of the modulo two adder 38 is a binary signal whosetransitions represent transitions in the output of the amplifier 154 atthe transmitter (FIG. 1 i.e., the output of the adder 38 is a homologueof the output of the amplifier 14. This arises from the fact that abinary number which is modulo two added twice to another number givesthe other number; i.e., even numbers of modulo two additions of anybinary number or sequence are self-cancelling.

The binary output of the adder 38 is combined with the amplitudeenvelope signal from the low pass filter 36 in a suitable combinationcircuit 42 (which will be an amplitude modulator if the circuit 24 is anamplitude modulator); integrated in an integrator 44 to cancel out theeffects of the differentiator 12 (FIG. 1); filtered in a low pass filter46 to remove the noise and spurious high frequency components introducedfor example by the limiter 32, the sequence generator 40, and the adder38; and converted to audible signals in a suitable transducer 48 such asheadphones or a loudspeaker. The output of the transducer 48 will be anintelligible reproduction of the speech or other data received by themicrophone (FIG. 1).

The pseudo-randon sequence generators l8 and 40 may be synchronised inany convenient manner. Where the generators '18 and 40 are shiftregisters whose inputs are the outputs of respective exclusive OR gatesfed from, say, the last stage and one other stage of each register, (aknown fomr of pseudo-random binary sequency generator), the followingsynchronising procedure may be followed. With a speech-free signal beingreceived, the input of the shift register in the sequence generator 40is disconnected and the output of the amplifier 32 substituted. Thiscauses the shift register to take up the portion of the sequencecurrently being generated at the transmitter terminal, and so primes theregister correctly. The output of the exclusive OR gate within thegenerator 40 is compared with the input to the register and when theyhave maintainedidentity for a suitable length of time (i.e., the timerequired for the register to be primed (equal to the time required toclock one bit through the length of the register) plus any desiredsafety margin) the generators l8 and 40 are assumed to be insynchronism. The amplifier 32 is then disconnected from the register andreconnected as shown in FIG. 3, and the exclusive OR gate reconnected tothe input of the register. The sequence generator 40 then continues in aself-sustaining mode. Register clocking signals may be obtained at thereceiver in any known manner from the broadcast signal to maintainsynchronism of the respective generators. It can be arranged that theabove procedure is carried out automatically at commencement ofreception, or manually performed when required. (It is assumed of coursethat the connections to the exclusive OR gates are the same in thetransmitter and receiver terminals, i.e., that the correct code of theday" is being used).

The larger the number of zero crossings, the better will be the qualityof the reconstituted speech or other date at the receiver; hence theinclusion of the differentiator 12 in the transmitter to produce agreater number of zero crossings in the data to be transmitted. Theeffect of the differentiator 12 is substantially nullified at thereceiver by the integrator 44. The data transmission system of FIGS. 1and 2 will function without zerocrossing enhancement i.e., if thedifferentiator 12 and the integrator 44 were omitted, but transmissionquality will be somewhat poorer.

The transmitted signals in the system of FIGS. 1 and 2 may be summarisedas being the true envelope and scrambled zero-crossings. The transmittedsignals of the system now to be described with reference to FIGS. 3 and4 may be summarised as being scrambled envelope and scrambledzero-corssings.

Referring to FIG. 3, the transmitter terminal apparatus of the systemagain comprises the microphone l0 and the differentiator 12. A firstpseudo-random binary sequence from a first pseudo-random binary sequencegenerator 52 is linearly added to the output of the differentiator 12 ina suitable analogue linear addition circuit 50. The first sequence is ata very low amplitude level compared to the output of the differentiator12. The combined signal from the addition circuit 52 is then passedthrough a high gain limiting amplifier 54 whose output is a binarysignal wherein transitions represent zero-crossings in the input to theamplifier 54.

The binary output from the amplifier 54 is applied to a modulo two adder56 as one input thereof. The other input to the adder 56 is a secondpseudo-random binary sequence from a second pseudo-random binarysequence generator 58. The output of the modulo two adder 56 isbroadcast by the transmitter 26, and is completely unintelligiblewithout knowledge of both the first and second pseudo-random sequences.A filter (not shown) may again be provided between the adder 56 and thetransmitter 26.

A form of receiver terminal apparatus suitable for receiving andunscrambling data transmitted by the transmitter apparatus of FIG. 3 isshown in FIG. 4 to which reference will now be made. The output of thereceiver 30 is converted to a binary sequence by a high gain limitingamplifier 60, and this binary sequence forms one input of a modulo twoadder 62. The other input to the adder 62 is the second pseudo-randomsequence produced by a sequence generator 64 arranged to produce thesame sequence as the generator 58 (FIG. 3) and synchronised therewith.The output of the adder 62 is passed to a further modulo two adder 66whose other input is the first pseudo-random sequence, produced by asequence generator 68 arranged to produce the same sequence as thegenerator 52 (FIG. 3) and synchronised therewith. The output of theadder 66 is passed through a low-pass filter 70, a non-linear circuit 72which corrects for the inherent non-linearity of the amplitude channelcoding, and thence to the combination circuit 42, the integrator 44, thelow pass filter 46, and the output transducer 48 (c.f. FIG. 2). Thearrangement of FIGS. 3 and 4 functions as follows:

In the absence of a speech input to the microphone 10 (FIG. 3), thesignal transmitted by the transmitter 26 is the modulo two addition ofthe first and second sequences. With a very high amplitude speech input,the limiting amplifier 54 is captured by it, and the first sequence issuppressed from the output of the amplifier 54 (This arises from theconstancy of the power output of the amplifier 54). Thus the transmittedsignal tends towards the limiting case of the modulo two addition of thespeech input and the second sequence, as the amplitude of the speechincreases.

In the receiver (FIG. 4) the received signal (the output of the receiver30) is limited or sliced in the limiting amplifier 60 and then modulotwo added to the second sequence. The output of the first adder 62 is inturn modulo two added to the first sequence in the second modulo twoadder 66. In the absence of a speech representative signal in thereceived signal, the output of the second adder 66 is all zero," but asthe speech level increases and the first sequence is suppressed, theoutput of the second adder 66 contains an increasing amount of onelevel, until with an infinitely large speech signal, the output of theadder 66 is 50 percent one level. When the output of the adder 66 ispassed through the low pass filter 70 and the non-linear circuit 72, theenvelope component of the speech is obtained. The combination in theamplitude modulator circuit 42 of this envelope signal, and thezero-crossing signal from the adder 62 results in a signal, which whenintegrated in the integrator 44 and low pass filtered in the filter 46(for reasons given with reference to FIG.2), causes the outputtransducer 48 to give an intelligible reproduction of the speechreceived by the microphone 10.

In the above described arrangements, a greater degree of scrambling maybe obtained by transmitting signals representative of the conjugate, orinverse, of the amplitude envelope, rather than of the actual envelope.This has the additional advantage that it becomes much more difficult,if not impossible, to determine when speech is being transmitted, whichwould otherwise be indicated by bursts of noise. Furthermore, there is asignal continuously available for synchronising transmitter and receiversequence generators.

Although the embodiments above described refer only to two-level orbinary signals (when not referring to analogue signals), other numbersof levels in the multi-level signals may be employed; for example threeand four level signals may be employed. In these cases, subtractioncircuits are required at the receivers in place of the modulo-twoadders. (Conversely, subtraction of the pseudo-random sequences may beperformed at the transmitters with corresponding addition at thereceivers).

Signals other than speech signals may be transmitted as above described,and transmission may be by means other than wireless transmission, forexample, by line transmission, as in a telephone system.

Other modifications and variations are possible within the scope of theinvention.

1 claim:

1. Receiver terminal apparatus for receiving an input signal containingintelligence and delivering as an output signal the intelligencecontained in the input signal, the input signal comprising superimposedfirst and secnd components, said first component being the scalarcombination of a first digital signal having a plurality of possibleamplitude levels which occur in a pseudorandom sequence and a seconddigital signal having a plurality of possible amplitude levels whereinamplitude level transitions represent amplitude datum level transitionsin an analogue signal containing said intelligence, said secondcomponent representing the amplitude envelope of said analogue signalcontaining said intelligence, said receiver terminal apparatus comprismga. first signal generator means to generate a first replica signal whichis a replica of said first digital signal,

b. second signal generator means responsive to said input signal togenerate a second digital signal which has a plurality of possibleamplitude levels and in which transitions between said amplitude levelsrepresent traversals of at least one amplitude datum level by said inputsignal,

c. first signal combining means to combine said first replica signalwith said second digital signal to derive a second replica signalrepresenting said amplitude datum level transitions in said analoguesignal,

d. signal processing means to recover a third replica signal which is areplica of said amplitude envelope, and

e. second signal combining means to combine said third replica signalwith said second replica signal to form said output signal.

2. Apparatus according to claim 1 in which said signal processing meanscomprises amplitude envelope detector means coupled to receive saidinput signal and coupled to said second signal combining means in signalbypass relationship both to said second signal generator means and tosaid first signal combining means.

3. Apparatus according to claim 1 in which said signal processing meanscomprises third signal generator means to generate a third digitalsignal which has a plurality of possible amplitude levels which occur inpseudo-random sequence, third signal combining means to combine saidthird digital signal with said second digital signal to form a fourthdigital signal, and amplitude envelope derivation means coupled toreceive said fourth digital signal and to produce said third replicasignal therefrom.

4. A speech descrambler comprising a. means to receive a scrambledspeech signal,

b. signal squaring means to square the received scrambled speech signal,

c. a pseudo-random sequence generator,

d. first signal combining means to combine the output of the squaringmeans with the output of the pseudo-random sequence generator,

e. a further pseudo-random sequence generator,

f. second signal combining means to combine the output of the firstsignal combining means with the output of the further pseudo-randomsequence generator,

g. amplitude envelope derivation means to derive the amplitude envelopefrom the output of said second signal combining means,

h. third signal combining means to combine the output of said firstsignal combining means with the output of said amplitude envelopederivation means to produce a descrambled speech signal, and

. audio transducer means to convert said descrambled speech signal toaudible descrambled speech.

5. A speech descrambler according to claim 4 in which said amplitudeenvelope derivation means comprises series-connected low-pass filtermeans and nonlinear circuit means.

6. A speech descrambler according to claim 4 further includinghigh-frequency de-emphasizing means coupled between said third signalcombining means and said transducer means.

7. A speech descrambler according to claim 6 in which saidhigh-frequency de-emphasizing means comprises signal integrator means.

8. A speech descrambler according to claim 4, further including noiseand spurious high-frequency signal component suppression means coupledbetween said third signal combining means and said transducer means.

9. A speech descrambler according to claim 8, in which said suppressionmeans is a low-pass filter.

10. A communication system for transmitting the intelligence of anintelligence containing signal having components of variable frequencyand variable amplitude, said intelligence containing signal frequentlytraversing at least one amplitude datum level including zero amplitude,said intelligence containing signal having an amplitude envelope, saidsystem comprising:

i. a transmitter terminal;

ii. a receiver terminal; and

iii. a communication path between said transmitter and receiverterminals;

iv. said transmitter terminal comprising:

a. first signal generator means to generate a first digital signalhaving a plurality of possible amplitude levels which occur inpseudo-random sequence;

b. second signal generator means responsive to said intelligencecontaining signal to generate a second digital signal which has aplurality of possible amplitude levels and in which transitions betweensaid amplitude levels represent traversals of said at least oneamplitude datum level by said intelligence containing signal;

first signal processing means comprising a first scalar combinationmeans to effect scalar combination of the first digital signal and thesecond digital signal to form a first component of an output signal ofthe transmitter terminal;

d. second signal processing means comprising amplitude envelopeprocessing means for processing said amplitude envelope separately fromsaid second digital signal to form a second component of the outputsignal of the transmitter terminal;

e. means to include said second component in the output signal of saidtransmitter terminal with said first component; and

f. means to transmit said output signal along said communication path tosaid receiver terminal;

. said receiver terminal comprising:

a. means to receive the transmitter terminal output signal as a receiverterminal input signal;

b. third signal generator means to generate a first replica signal whichis a replica of said first digital signal;

. fourth signal generator means responsive to said receiver terminalinput signal to generate a third digital signal which has a plurality ofpossible amplitude levels and in which transitions between saidamplitude levels represent traversals of at least one amplitude datumlevel by said receiver terminal input signal;

d. first signal combining means to combine said first replica signalwith said third digital signal to derive a second replica signalrepresenting said amplitude datum level transitions in said intelligencecontaining signal;

. third signal processing means to recover a third replica signal whichis a replica of said amplitude envelope; and second signal combiningmeans to combine said third replica signal with said second replicasignal to form an output signal containing the intelligence that was insaid intelligence containing signal.

l l. A system according to claim 10 in which said second signalprocessing means comprises bypass means to couple the amplitude envelopesignal directly to said means to include said second component in thetransmitter terminal output signal, in signal bypass relationship bothto the second signal generator means and to the first signal processingmeans.

12. A system according to claim 10 in which the amplitude envelopeprocessing means of said second signal processing means comprises afifth signal generator means to generate a fourth digital signal whichhas a pluraity of possible amplitude levels which occur in pseudo-randomsequence, and a further combination means to effect combination of thefourth digital signal with the amplitude envelope signal to form saidsecond component.

13. A system according to claim 10 including circuit means through whichthe input signal is passed to increase the amplitude datum leveltransition rate.

14. A system according to claim 11 in which said third signal processingmeans comprises amplitude envelope detector means coupled to receivesaid receiver terminal input signal and coupled to said second signalcombining means in signal bypass relationship both to said fourth signalgenerator means and to said first signal combining means.

15. A system according to claim. 12 in which said third signalprocessing means comprises sixth signal generator means to generate afifth digital signal which has a plurality of possible amplitude levelswhich occur in pseudo-random sequence, third signal combining means tocombine said fifth digital signal with said third digital signal to forma sixth digital signal, and amplitude envelope derivation means coupledto receive said sixth digital signal and to produce said third replicasignal therefrom.

16. A speech scrambler/descrambler system comprising microphone means, afirst pseudo-random sequence generator, first signal combining means tocombine the output of the microhphone means with the output of the firstpseudo-random sequence generator, first signal squaring means to squarethe output of said first signal combining means, a second pseudo-randomsequence generator, second signal combining means to combine the outputof the signal squaring means with the output of the second pseudo-randomsequence generator to produce a scrambled speech signal, means toreceive the scrambled speech signal, second signal squaring means tosquare the received scrambled speech signal, a third pseudo-randomsequence generator, third signal combining means to combine the outputof the second squaring means with the output of the third pseudo-randomsequence generator, a fourth pseudo-random sequence generator, fourthsignal combining means to combine the output of the third signalcombining means with the output of the fourth pseudorandom seqencegenerator, amplitude envelope derivation means to derive the amplitudeenvelope from the output of said fourth signal combining means, fifthsignal combining means to combine the output of said third signalcombining means with the output of said amplitude envelope derivationmeans to produce a descrambled speech signal, and audio transducer meansto convert said descrambled speech signal to audible descrambled speech.

17. A system according to claim 16 in which highfrequency enphasizingmeans is coupled between said microphone means and said first signalcombining means.

18. A system according to claim 16 in which said amplitude envelopederivation means comprises seriesconnected low-pass filter means andnon-linear circuit means.

19. A system according to claim 16 further including high-frequencyde-emphasizing means coupled between said fifth signal combining meansand said transducer means.

20. A system according to claim 16 further including noise and suprioushigh-frequency signal component suppression means coupled between saidfifth signal combining means and said transducer means.

1. Receiver terminal apparatus for receiving an input signal containingintelligence and delivering as an output signal the intelligencecontained in the input signal, the input signal comprising superimposedfirst and second components, said first component being the scalarcombination of a first digital signal having a plurality of possibleamplitude levels which occur in a pseudo-random sequence and a seconddigital signal having a plurality of possible amplitude levels whereinamplitude level transitions represent amplitude datum level transitionsin an analogue signal containing said intelligence, said secondcomponent representing the amplitude envelope of said analogue signalcontaining said intelligence, said receiver terminal apparatuscomprising a. first signal generator means to generate a first replicasignal which is a replica of said first digital signal, b. second signalgenerator means responsive to said input signal to generate a seconddigital signal which has a plurality of possible amplitude levels and inwhich transitions between said amplitude levels represent traversals ofat least one amplitude datum level by said input signal, c. first signalcombining means to combine said first replica signal with said seconddigital signal to derive a second replica signal representing saidamplitude datum level transitions in said analogue signal, d. signalprocessing means to recover a third replica signal which is a replica ofsaid amplitude envelope, and e. second signal combining means to combinesaid third replica signal with said second replica signal to form saidoutput signal.
 2. Apparatus according to claim 1 in which said signalprocessing means comprises amplitude envelope detector means coupled toreceive said input signal and coupled to said second signal combiningmeans in signal bypass relationship both to said second signal generatormeans and to said first signal combining means.
 3. Apparatus accordingto claim 1 in which said signal processing means comprises third signalgenerator means to generate a third digital signal which has a pluralityof possible amplitude levels which occur in pseudo-random sequence,third signal combining means to combine said third digital signal withsaid second digital signal to form a fourth digital signal, andamplitude envelope derivation means coupled to receive said fourthdigital signal and to produce said third replica signal therefrom.
 4. Aspeech descrambler comprising a. means to receive a scrambled speechsignal, b. signal squaring means to square the received scrambled speechsignal, c. a pseudo-random sequence generator, d. first signal combiningmeans to combine the output of the squaring means with the output of thepseudo-random sequence generator, e. a further pseudo-random sequencegenerator, f. second signal combining means to combine the output of thefirst signal combining means with the output of the furtherpseudo-random sequence generator, g. amplitude envelope derivation meansto derive the amplitude envelope from the output of said second signalcombining means, h. third signal combining means to combine the outputof said first signal combining means with the output of said amplitudeenvelope derivation means to produce a descrambled speech signal, and i.audio transducer means to convert said descrambled speech signal toaudible descrambled speech.
 5. A speech descrambler according to claim 4in which said amplitude envelope derivation means comprisesseries-connected low-pass filter means and non-linear circuit means. 6.A speech descrambler according to claim 4 further includinghigh-frequency de-emphasizing means coupled between said third signalcombining means and said transducer means.
 7. A speech descrambleraccording to claim 6 in which said high-frequency de-emphasizing meanscomprises signal integrator means.
 8. A speech descrambler according toclaim 4, further including noise and spurious high-frequency signalcomponent suppression means coupled between said third signal combiningmeans and said transducer means.
 9. A speech descrambler according toclaim 8, in which said suppression means is a low-pass filter.
 10. Acommunication system for transmitting the intelligence of anintelligence containing signal having components of variable frequencyand variable amplitude, saId intelligence containing signal frequentlytraversing at least one amplitude datum level including zero amplitude,said intelligence containing signal having an amplitude envelope, saidsystem comprising: i. a transmitter terminal; ii. a receiver terminal;and iii. a communication path between said transmitter and receiverterminals; iv. said transmitter terminal comprising: a. first signalgenerator means to generate a first digital signal having a plurality ofpossible amplitude levels which occur in pseudo-random sequence; b.second signal generator means responsive to said intelligence containingsignal to generate a second digital signal which has a plurality ofpossible amplitude levels and in which transitions between saidamplitude levels represent traversals of said at least one amplitudedatum level by said intelligence containing signal; c. first signalprocessing means comprising a first scalar combination means to effectscalar combination of the first digital signal and the second digitalsignal to form a first component of an output signal of the transmitterterminal; d. second signal processing means comprising amplitudeenvelope processing means for processing said amplitude envelopeseparately from said second digital signal to form a second component ofthe output signal of the transmitter terminal; e. means to include saidsecond component in the output signal of said transmitter terminal withsaid first component; and f. means to transmit said output signal alongsaid communication path to said receiver terminal; v. said receiverterminal comprising: a. means to receive the transmitter terminal outputsignal as a receiver terminal input signal; b. third signal generatormeans to generate a first replica signal which is a replica of saidfirst digital signal; c. fourth signal generator means responsive tosaid receiver terminal input signal to generate a third digital signalwhich has a plurality of possible amplitude levels and in whichtransitions between said amplitude levels represent traversals of atleast one amplitude datum level by said receiver terminal input signal;d. first signal combining means to combine said first replica signalwith said third digital signal to derive a second replica signalrepresenting said amplitude datum level transitions in said intelligencecontaining signal; e. third signal processing means to recover a thirdreplica signal which is a replica of said amplitude envelope; and f.second signal combining means to combine said third replica signal withsaid second replica signal to form an output signal containing theintelligence that was in said intelligence containing signal.
 11. Asystem according to claim 10 in which said second signal processingmeans comprises bypass means to couple the amplitude envelope signaldirectly to said means to include said second component in thetransmitter terminal output signal, in signal bypass relationship bothto the second signal generator means and to the first signal processingmeans.
 12. A system according to claim 10 in which the amplitudeenvelope processing means of said second signal processing meanscomprises a fifth signal generator means to generate a fourth digitalsignal which has a pluraity of possible amplitude levels which occur inpseudo-random sequence, and a further combination means to effectcombination of the fourth digital signal with the amplitude envelopesignal to form said second component.
 13. A system according to claim 10including circuit means through which the input signal is passed toincrease the amplitude datum level transition rate.
 14. A systemaccording to claim 11 in which said third signal processing meanscomprises amplitude envelope detector means coupled to receive saidreceiver terminal input signal and coupled to said second signalcombining means in signal bypass relationship both to said fourth signalgenerator means and to said first sIgnal combining means.
 15. A systemaccording to claim 12 in which said third signal processing meanscomprises sixth signal generator means to generate a fifth digitalsignal which has a plurality of possible amplitude levels which occur inpseudo-random sequence, third signal combining means to combine saidfifth digital signal with said third digital signal to form a sixthdigital signal, and amplitude envelope derivation means coupled toreceive said sixth digital signal and to produce said third replicasignal therefrom.
 16. A speech scrambler/descrambler system comprisingmicrophone means, a first pseudo-random sequence generator, first signalcombining means to combine the output of the microhphone means with theoutput of the first pseudo-random sequence generator, first signalsquaring means to square the output of said first signal combiningmeans, a second pseudo-random sequence generator, second signalcombining means to combine the output of the signal squaring means withthe output of the second pseudo-random sequence generator to produce ascrambled speech signal, means to receive the scrambled speech signal,second signal squaring means to square the received scrambled speechsignal, a third pseudo-random sequence generator, third signal combiningmeans to combine the output of the second squaring means with the outputof the third pseudo-random sequence generator, a fourth pseudo-randomsequence generator, fourth signal combining means to combine the outputof the third signal combining means with the output of the fourthpseudo-random seqence generator, amplitude envelope derivation means toderive the amplitude envelope from the output of said fourth signalcombining means, fifth signal combining means to combine the output ofsaid third signal combining means with the output of said amplitudeenvelope derivation means to produce a descrambled speech signal, andaudio transducer means to convert said descrambled speech signal toaudible descrambled speech.
 17. A system according to claim 16 in whichhigh-frequency enphasizing means is coupled between said microphonemeans and said first signal combining means.
 18. A system according toclaim 16 in which said amplitude envelope derivation means comprisesseries-connected low-pass filter means and non-linear circuit means. 19.A system according to claim 16 further including high-frequencyde-emphasizing means coupled between said fifth signal combining meansand said transducer means.
 20. A system according to claim 16 furtherincluding noise and suprious high-frequency signal component suppressionmeans coupled between said fifth signal combining means and saidtransducer means.